Impacts of meteorology and precursor emission change on O variation in Tianjin, China from 2015 to 2021.

Deterioration of surface ozone (O) pollution in Northern China over the past few years received much attention. For many cities, it is still under debate whether the trend of surface O variation is driven by meteorology or the change in precursors emissions. In this work, a time series decomposition method (Seasonal-Trend decomposition procedure based on Loess (STL)) and random forest (RF) algorithm were utilized to quantify the meteorological impacts on the recorded O trend and identify the key meteorological factors affecting O pollution in Tianjin, the biggest coastal port city in Northern China. After "removing" the meteorological fluctuations from the observed O time series, we found that variation of O in Tianjin was largely driven by the changes in precursors emissions. The meteorology was unfavorable for O pollution in period of 2015-2016, and turned out to be favorable during 2017-2021. Specifically, meteorology contributed 9.3 µg/m O (13%) in 2019, together with the increase in precursors emissions, making 2019 to be the worst year of O pollution since 2015. Since then, the favorable effects of meteorology on O pollution tended to be weaker. Temperature was the most important factor affecting O level, followed by air humidity in O pollution season. In the midday of summer days, O pollution frequently exceeded the standard level (>160 µg/m) at a combined condition with relative humidity in 40%-50% and temperature > 31°C. Both the temperature and the dryness of the atmosphere need to be subtly considered for summer O forecasting.